Detection of ochratoxin A producing fungi

ABSTRACT

Nucleotide or amino acid sequences that may be used in the detection and/or identification for an ochratoxigenic fungus or in the construction of an atoxigenic strain of an ochratoxigenis fungus. The fungus may be of the genus  Aspergillus , species  carbonarius, niger, alliaceus , or  foetidux . The fungus may also be of the genus  Penicillium , species  verrucosum.

This is a continuation of PCT/IE2004/000020 filed 12 Feb. 2004 and published in English.

INTRODUCTION

The invention relates to genes expressed by fungal cells producing the mycotoxin ochratoxin A (OTA) and a method for detecting ochratoxin A, particularly in food and feedstuffs.

BACKGROUND

Mycotoxins are a group of secondary metabolites which are produced by various filamentous fungi that can cause a toxic response termed a mycotoxicosis, if ingested by higher vertebrates and other animals.

Ochratoxin A (OTA) is a mycotoxin produced by Aspergillus and Penicillium species considered detrimental to human health and is classified as a possible human carcinogen. The recommended level for OTA in food for human consumption is 5 pg/kg for raw grain, 3 pg/kg for derived cereal products and 10 ilg/kg for dried vine fruits. The European Commission's Scientific Committee on Food has concluded that the intake of OTA should be reduced as far as possible to approximately 5 ng per kilogram of body weight per day (Scientific Committee for food 1996).

Cereals normally correspond to 50 to 80% of average consumer intake. OTA is found mainly in wheat and barley (Kuiper-Goodman 1996; Pittet 1998). It is also found in coffee (Burdaspal and Legarda, 1998; Dietrich et al., 1995, Jorgenson, 1998), wine (Burdaspal and Legarda, 1999; Visconti et al., 1999; Zimmerli and Dick, 1996), beer (Visconti et al., 2000), pork (Jorgenson, 1998; Wolff et al, 2000) and grapes.

Conventional methods for fungal detection and identification in a sample involve plating, incubation and identification based on morphological characteristics. These methods are time-consuming, labour intensive and require experienced personnel that may be lacking in many laboratories to interpret the results.

There is therefore a clear need for a more accessible and improved method for the detection and identification of the presence of OTA producing fungi present at even very low levels in a sample.

STATEMENTS OF INVENTION

According to the invention there is provided use of a gene which is expressed by fungal cells producing ochratoxin A, or a fragment, derivative, mutant or variant of the gene in the detection and/or identification of ochratoxigenic fungi.

The invention also provides use of a gene which is upregulated during ochratoxin A biosynthesis, or a fragment, derivative, mutant or variant of the gene in the detection and/or identification of ochratoxigenic fungi.

The invention further provides use of a protein having an amino acid sequence derived from any one or more of SEQ ID No 1, SEQ ID No 2, SEQ ID No 3, SEQ ID No 4, SEQ ID No 5, SEQ ID No 6 or SEQ ID No 7 or SEQ ID No 8 or SEQ ID No 9 in the detection and/or identification of an ochratoxin A producing fungi. The use may be in the detection and/or identification of fungi that express ochratoxin A biosynthetic genes.

In one aspect the invention provides use of a nucleotide sequence, gene, peptide or polypeptide or a fragment, derivative, or variant thereof having a nucleotide or amino acid sequence selected from any one or more of SEQ ID No. 1 or SEQ ID No. 2 or SEQ ID No. 3 or SEQ ID No. 4 or SEQ ID No. 5 or SEQ ID No. 6 or SEQ ID No. 7 or SEQ ID No 8 or SEQ ID No 9 in the detection and/or identification of an ochratoxigenic fungus.

The invention also provides use of a nucleotide sequence, gene, peptide or polypeptide or a fragment, derivative, or variant thereof having a nucleotide or amino acid sequence selected from any one or more of SEQ ID No. 1 or SEQ ID No. 2 or SEQ ID No. 3 or SEQ ID No. 4 or SEQ ID No. 5 or SEQ ID No. 6 or SEQ ID No. 7 or SEQ ID No. 8 or SEQ ID No. 9 in the construction of an atoxigenic strain of an ochratoxigenic fungus.

The ochratoxigenic fungus may be of the genus Aspergillus. The fungus may be of the species carbonarius, niger, alliaceus or foetidus.

Alternatively the ochratoxigenic fungus is of the genus Penicillium. The fungus may be of the species verrucosum.

The invention also provides:

An isolated nucleotide having SEQ ID No 1.

An isolated nucleotide having SEQ ID No 2.

An isolated nucleotide having SEQ ID No 3.

An isolated nucleotide having SEQ ID No 4.

An isolated nucleotide having SEQ ID No 5.

An isolated nucleotide having SEQ ID No 6.

An isolated nucleotide having SEQ ID No 7.

An isolated nucleotide having SEQ ID No 8.

An isolated nucleotide having SEQ ID No 9.

The invention also provides Oligonucleotide primers derived from any of these isolated nucleotides.

More particularly the invention provides PCR primers prepared derived from any of these isolated nucleotides.

The invention also provides an oligonucleotide primer selected from one or more of:

SEQ Nucleotide ID Parent SEQ Position in No. Primer ID No. Parent Sequence 10 AOB02-F: 5′-tatccgccgcctcgcaaactaat-3′ SEQ ID No. 6 11 AOB02-R: 5′-cgaccgatcatgcgaccgtaat-3′ SEQ ID No. 6 12 AOB03-R: 5′-ctcggtgacatcaggggtatc-3′ SEQ ID No. 5 949-969 13 AOB03-R: 5′-agcgtattcagtcactcattcaga-3′ SEQ ID No. 5 14 AOE04-F: 5′-gctatgcgcggagaagtca-3′ SEQ ID No. 2 804-822 15 AOE04-R: 5′-aaggctggggatcgtggagtg-3′ SEQ ID No. 2 1605-1085 16 AOD07-F: 5′-agtttaccggccttgttga-3′ SEQ ID No. 4 17 AOD07-R: 5′-ttattaccgtttgtcgctcttctc-3′ SEQ ID No. 4 18 AOH11-F: 5′-agaacgggatgccaaaacagtgag-3′ SEQ ID No. 1 19 AOH11-R: 5′-aagaatgcgagggatgggataacc-3′ SEQ ID No. 1 20 2B11-BF: 5′-ttctctactgcgcttctcacatccat-3′ SEQ ID No. 7 2755-2780 21 2B11-BR: 5′-aacatcatagccataagaggtcaaca-3′ SEQ ID No. 7 2963-2988 22 PKS4-GAPF: 5′-agccgtgttttcattctttc-3′ SEQ ID No. 7 1610-1629 23 PKS4-GAPR: 5′-tgcggccatcttcgtgt-3′ SEQ ID No. 7 2346-2362 24 KS-DPA: 5′-GCIAAYGGITAYGCIMGIGG-3′ SEQ ID No. 8 25 KS-DPB- 5′-GTICCIGTICCRTAIGCYTC-3′ SEQ ID No. 8 26 ACKS-1F: 5′-tgggtatgcgcggggtgagggtat-3′ SEQ ID No. 8 27 ACKS-1R: 5′-ccgtaggcttcgaaaaactgacac-3′ SEQ ID No. 8

The invention further provides an Oligonucleotide primer selected from one or more of:

SEQ ID No. Primer Parent SEQ ID No. 24 KS-DPA: 5′-GCIAAYGGITAYGCIMGIGG-3′ SEQ ID No. 9 25 KS-DPB- 5′-GTICCIGTICCRTAICGYTC-3′ SEQ ID No. 9 28 PVKS-1F: 5′-tgcacgaccgggacaacatca-3′ SEQ ID No. 9 29 PVKS-1R: 5′-ccgtaggcctccacaaaatctg-3′ SEQ ID No. 9

In another aspect the invention provides an assay for the detection of ochratoxin A producing fungi comprising a nucleotide sequence, gene, peptide or polypeptide or a fragment, derivative, or variant thereof having a nucleotide or amino acid sequence selected from any one or more of SEQ ID No. 1 or SEQ ID No. 2 or SEQ ID No. 3 or SEQ ID No. 4 or SEQ ID No. 5 or SEQ ID No. 6 or SEQ ID No. 7 or SEQ ID No 8 or SEQ ID No 9.

The invention also provides an assay for the detection and identification of ochratoxin A producing genes comprising a nucleotide sequence, gene, peptide or polypeptide or a fragment, derivative, or variant thereof having a nucleotide or amino acid sequence selected from any one or more of SEQ ID No. 1 or SEQ ID No. 2 or SEQ ID No. 3 or SEQ ID No. 4 or SEQ ID No. 5 or SEQ ID No. 6 or SEQ ID No. 7 or SEQ ID No 8 or SEQ ID No 9.

In a further aspect the invention provides an atoxigenic strain of the genus Aspergillus. The atoxigenic strain may be of the species ochraceus, carbonarius, alliaceus, foetidus or niger.

In another aspect the invention provides an atoxigenic strain of the genus Penicillium. The atoxigenic strain may be of the species verrucosum.

The atoxigenic strain may comprise a nucleotide sequence, gene, peptide or polypeptide or a fragment, derivative, or variant thereof having a nucleotide or amino acid sequence selected from any one or more of SEQ ID No. 1 or SEQ ID No. 2 or SEQ ID No. 3 or SEQ ID No. 4 or SEQ ID No. 5 or SEQ ID No. 6 or SEQ ID No. 7 or SEQ ID No 8 or SEQ ID No 9.

The invention further provides use of a nucleotide sequence, gene, peptide or polypeptide or a fragment, derivative, or variant thereof having a nucleotide or amino acid sequence selected from any one or more of SEQ ID No. 1 or SEQ ID No. 2 or SEQ ID No. 3 or SEQ ID No. 4 or SEQ ID No. 5 or SEQ ID No. 6 or SEQ ID No. 7 or SEQ ID No. 8 or SEQ ID No. 9 to identify or isolate a nucleotide sequence, gene, peptide or polypeptide involved in ochratoxin A biosynthesis. The sequences may be used for so-called chromosome walking or gene library screening experiments to obtain other DNA sequences contiguous to the sequences involved in OTA production.

BRIEF DESCRIPTION OF THE DRAWINGS

The invention will be more clearly understood from the following descriptions thereof, given by way of example only, with reference to the accompanying drawings, in which: —

FIGS. 1 (a) to (e) show the results of RT-PCR on the expression of cloned genes during OTA production by Aspergillus ochraceus HP99. Expression of Glyceraldehyde-3-phosphate dehydrogenase (G3PDH) (a) is used as a control. Samples were taken at 4, 5, 6, 7, 8 and 10 day time intervals;

FIG. 2 shows the use of a PCR primer pair AOKS-RT2 (SEQ ID No. 30 and 31) to distinguish between genomic and cDNA. The 2B11 primer reactions are included for comparison.

FIG. 3 shows the results of thin layer chromatography (TLC) at each time interval;

FIG. 4 shows the results of a plate screening confirming the involvement of the pks gene in OTA biosynthesis. The blue fluorescent halo is indicated by an arrow.

FIG. 5 shows the results of TLC on atoxigenic strains of A. ochraceus (AO118 [lane 5]; A0107 [lane 4]; A053 [lane 3]) in comparison to ochratoxin A standards (lanes 1 and 7) and wild type A ochraceus extract (lanes 2 and 6). The position of OTA is indicated by the arrow.

FIG. 6 Shows RT-PCR analysis expression of the pAs gene in Aspergillus foetidus ATCC 10254, using the 2B11 primer pair (SEQ ID No. 20 and 21). During growth on (1) malt extract broth, (2) potato dextrose broth and (3) yeast extract glucose broth. Lanes 1, 2 and 3 are G3PDH controls. Lanes 1′, 2′ and 3′ are the 2B11 primer reactions. +VE is positive control, −VE are negative controls.

FIG. 7 Shows production of OTA by A. foetidus ATCC10254 mycelium from which the RNA for the above RT-PCR experiment was extracted.

FIG. 8 Shows RT-PCR measurement of expression of the pks gene by Aspergillus niger ATCC9029 using the GAP primer pair (SEQ ID No. 22 and 23). During growth on (1) malt extract broth, (2) potato dextrose broth and (3) yeast extract glucose broth. Lanes 1, 2 and 3 are G3PDH controls. Lanes 1′, 2′ and 3′ are the 2B11 primer reactions. +VE is positive control, −VE are negative controls.

DETAILED DESCRIPTION

The invention provides isolated nucleotide sequences and genes expressed by fungal cells producing OTA and believed to be involved in the biosynthetic pathway for OTA biosynthesis.

The invention also provides detection methods based on these isolated nucleotide sequences to detect the presence of fungi producing OTA in a sample.

The detection methods of the invention provide simple and more rapid assays for the detection of ochratoxigenic fungi and have significant advantages over the detection methods currently available.

To date the biosynthetic pathway for OTA biosynthesis has not been characterised. However, the cloning and characterisation of the genes of the invention greatly increases the understanding of the biosynthetic pathway. The cloning and molecular characterisation of mycotoxin biosynthetic genes is vital in order to gain a fuller understanding of the organisation, regulation and expression of these genes.

In the invention suppression substractive hybridisation PCR (SSH-PCR) was used to clone a pool of cDNA sequences from Aspergillus ochraceus HP99 (University College Cork, Dept. of Microbiology, Culture Collection) that are expressed at higher levels during OTA biosynthesis. The cloned DNA sequences were then compared to sequences deposited in various databases in order to identify putative OTA biosynthetic genes.

A number of novel cloned DNA sequences were found to be expressed only by fungal cells producing OTA.

Polymerase chain reaction (PCR) based detection methods have the potential to detect and identify the present of mycotoxigenic fungi present at very low levels.

(Geisen, 1998; Edwards et al., 2002).

Using the novel DNA sequences of the invention oligonucleotide primer pairs of the identified sequences were prepared. The oligonucleotide primer pairs form the basis of two nucleic acid based detection methods of the invention.

The detection methods of the invention use PCR and are extremely specific to the particular genus/species. They provide rapid detection using techniques which are easily learnt. The methods are based on the identification and isolation of the novel gene sequences of the invention which have been found to be specific to OTA.

In the first method a conventional PCR assay is used to detect fungi that have the capacity to produce OTA by indicating the presence of OTA biosynthesis genes in fungal isolates. DNA from fungal isolates may be used as a template in PCR reactions. Oligonucleotide primers derived from the cloned gene sequences detect fungal isolates that are capable of producing OTA. Modern Real-time PCR based protocols can provide results in less than 40 minutes.

In the second method a reverse transcription (RT)-PCR based assay is used to identify fungi that are expressing the OTA biosynthetic genes and are therefore toxigenic. The RT-PCR assay detects the level of transcription (production of messenger RNA, mRNA) for a specific gene. The assay is performed in two stages (1) synthesis of cDNA from mRNA and (2) amplification of specific target sequences by PCR. The RT-PCR based assay provides highly specific detection of mRNA transcripts from the targeted genes. As RNA synthesis is the primary step in producing the enzymes necessary for any biosynthetic pathway the highly sensitive PCR based method demonstrates toxigenicity even when the amount of toxin produced is not detectable by conventional assays.

The isolation of the genes of the invention therefore has significant commercial benefit. The isolated DNA sequences may be used in a variety of assays for the detection of fungi producing ochratoxin A.

The PCR based detection system will detect the producing organism thereby allowing identification of potential problems from mycotoxin contamination prior to any significant amounts of toxin being produced.

A potential market for assays which can detect even the smallest amount of OTA producing fungi are the brewing, baking, wine making or animal feed industries as well as coffee manufacturers and cereal producers. The detection of OTA producing fungi or the presence of genes producing OTA at an early stage would ensure that the contaminated produce does not enter the food chain at source. Positive detection would lead to disposal of the contaminated produce and/or treatment to remove it.

The identification and isolation of the genes involved in the biosynthesis of OTA has valuable therapeutic potential. Mycotoxins are known to be involved in a number of disease states including cancer of the liver, damage of kidneys, weakening of the immune system, allergic reactions, ergotism and poisoning. Easy to use and rapid PCR based methods by which OTA producing fungi may be detected in source samples is therefore highly desirable.

The identification and characterisation of OTA producing genes is also important in determining the conditions under which the fungus produces the mycotoxin. The RT-PCR assay allows identification of the physiological parameters that promote ochratoxin production during storage such as water activity, temperature, pH etc. Storage conditions that are unfavourable to ochratoxin production can then be used to reduce or eliminate ochratoxin production in stored products. Information on the conditions that inhibit ochratoxin production are also essential to the use of HACCP to ensure mycotoxin-free food and animal feed.

The genes of the invention may also be used in the construction of atoxigenic strains of A. ochraceus, A. carbonarius, A. alliaceus, A. niger, A. foetidus, or Penicillium verrucosum. Atoxigenic strains are strains which are genetically engineered not to produce OTA. These strains may therefore be used in biocontrol strategies for the elimination of OTA production. In this way atoxigenic stains of fungi may be used without the risk of OTA being produced.

The genes of the invention may also be used to determine whether strains of Aspergillus strains which are used in industrial fermentations possess ochratoxin A biosynthetic genes i.e. the ability to produce the mycotoxin.

The genes of the invention may also be used to monitor whether OTA is being produced in industrial fermentations involving Aspergillus strains.

The invention will be more clearly understood from the following examples.

Methodology

The method of SSH-PCR used is described in detail by Diatchenko et al. (1996) and a commercially available kit for the procedure is produced by Clontech laboratories (Palo Alto, Calif., USA).

(1) Development of Permissive and Restrictive Growth Media

The growth conditions under which Aspergillus ochraceus HP99 produced OTA (permissive) and did not produce OTA (restrictive) were identified. A number of different growth media were examined and it was observed that OTA was not produced when A. ochraceus was grown on Potato dextrose medium (PD) or malt extract medium (ME) either as liquid medium or solid medium. High levels of OTA production were observed if A. ochraceus was grown on yeast extract sucrose (YES) or Czapek-Dox medium containing yeast extract and casamino acids (MC).

(2) SSH-PCR Procedure

Total ribonucleic acid (RNA) was isolated from A. ochraceus HP99 mycelia grown on permissive (MC) and restrictive (PD) media. Double stranded complementary DNA (cDNA) was then synthesised from each RNA preparation using a SMART™ cDNA synthesis kit (Clontech Laboratories, Palo Alto, Calif., USA) and used to perform a suppression-subtractive hybridisation-PCR experiment (SSH-PCR) experiment. The success of the subtractive hybridisation in eliminating DNA sequences common to both samples was determined by quantitative PCR on the SSH-PCR product using primers to the constitutively expressed housekeeping gene glyceraldehyde-3-phosphate dehydrogenase (G3PDH). The G3PDH gene was detectable at 5-10 fewer PCR cycles in the control sample, demonstrating a reduction in abundance of between 2⁵ and 2¹⁰ fold in the subtracted sample.

(3) Cloning Screening and Sequencing of SSH-PCR Products.

The PCR products resulting from the SSH-PCR were ligated into the pGEM-T easy vector (Promega Laboratories, Madison, Wis., USA) and the ligations transformed into Escherichia coli TOP 10 competent cells (Invitrogen). A large number of transformants were obtained and 1,440 were replica plated onto 155 mm LB agar plates.

To screen the replica plated clones for increased expression during OTA production a portion of each colony was picked and emulsified in 100, 1 of sterile water, zip of each colony suspension was then used as a template in a PCR reaction using the PCR primers used to amplify the final SSH-PCR products. The PCR products obtained from this procedure were spotted onto duplicate nitrocellulose membranes (2. 5. 1 per spot) and the DNA was fixed to the membranes by ultraviolet (UV) transillumination. The prepared membranes were then used in a hybridisation experiment to identify up-regulated clones. One membrane from each pair was probed with ³²P-ATP labelled cDNA from a permissive A. ochraceus culture and the other with ³²P-ATP labelled cDNA from a restrictive culture. Up-regulated clones were taken to be those for which the PCR product hybridised to the permissive probe but not to the restrictive probe. Clones that hybridised to both probes were disregarded. A total of 500 were screened of which 230 (46%) were upregulated (i.e. hybridised to permissive cDNA but not to restrictive cDNA). 192 up-regulated clones were selected for DNA sequencing. The nucleotide sequence of the cloned DNAs was determined and the deduced amino acid sequence of each sequence was compared to the protein sequence databases using the BLAST-X algorithm. The BLAST-X search translated the sequences in all 6 possible reading frames and compared them to the protein sequence databases (Table 1). 83 (55%) of the sequences were identifiable following the BLAST-X search. The largest number of identifiable clones encoded proteins with a role in cell metabolism, in addition about 30% of the sequences were classed as miscellaneous in that they could not be assigned to any of the several groupings. A number of clones with a potential role in ochratoxin biosynthesis were selected for further study. The nucleotide sequences of the selected clones are given in SEQ ID NO: 1-9.

(4) Further Studies on the Cloned Sequences

The presence of the cloned sequences in the genomic DNA of A. ochraceus and A. carbonarius 23804 was demonstrated by PCR using primers designed to the cloned sequences.

Reverse transcription-PCR was used to measure the expression of each of the cloned genes during OTA production.

TABLE 1 BLASTX hit size of clone (bp) Averantin oxidoreductase 630 HC toxin synthase 371 Regulatory gene 666 3-oxoacyl synthase 846 Trichodiene oxygenase 572 Acyl CoA dehydrogenase 670 Polyketide synthase 408

The most significant appears to be a gene encoding a protein sequence similar to a number of polyketide synthase (PKS) proteins. As the ochratoxin A molecule contains a polyketide structure a polyketide synthase gene is essential for its biosynthesis. The presence of all of the genes in the A. ochraceus genome was confirmed by PCR with primers designed to each of the cloned sequences. Most of the PCR primer pairs also amplified products of the correct size from the OTA producing fungus Aspergillus carbonarius 23804.

To confirm that the genes were up-regulated during OTA biosynthesis the RNA was isolated at a number of time points [4, 5, 6, 7, 8, and 10 days] from A. ochraceus cultures growing on permissive and restrictive medium and used in a RT-PCR experiment to estimate the expression level for each clone.

Reverse transcription-PCR (RT-PCR) studies of the expression of 4 of the selected clones under growth conditions permissive and restrictive for OTA production was carried out.

Total RNA was extracted from Aspergillus ochraceus mycelium at different time-points during growth. cDNA was prepared from each RNA sample and used as a template in a PCR reaction with primers specific to each of the selected genes. Primers to the glyceraldehyde 3-phosphate dehydrogenase gene that is constitutively expressed were used in control reactions. OTA production was measured at each time point [4, 5, 6, 7, 8, and 10 days] by thin-layer chromatography to confirm that the growth conditions were permissive or restrictive as appropriate (FIG. 2).

The reverse transcription step produces DNA complementary to all RNA molecules in a particular sample thus the presence of a product in the subsequent PCR reaction is evidence that the RNA transcript for a specific gene was present at a particular time point.

The RT-PCR assay detects the level of transcription (production of mRNA) for a specific gene. RNA was isolated from fungal mycelia under permissive (OTA being produced) and restrictive (OTA not being produced) conditions. The RNA transcripts were reverse transcribed (complementary DNA (cDNA) molecules were synthesised) by the enzyme reverse transcriptase. The cDNA molecules were then detected by PCR. In the example no (or very low levels) transcripts from the OTA biosynthetic genes were detected when OTA was not being synthesised (FIG. 1).

As shown in FIG. 1 the glyceraldehyde-3-phosphate gene was expressed in all of the samples in both the permissive and restrictive cultures. The SSH-PCR clones were expressed strongly only in the permissive cultures. It was noticeable that the expression levels were highest at the earlier timepoints when it was most likely that OTA was being actively synthesised.

The use of an intron-spanning primer pair (AOKS-RT2) (SEQ ID No. 30 and 31) (FIG. 1 (f)) provides additional assurance that it is cDNA that has been detected rather than genomic DNA contamination. The intron is absent from cDNA having been spliced out of the RNA transcript. The genomic DNA product (lane 3) is therefore larger than that from cDNA (lane 4). The 2B11 primer products (that do not span an intron) from the genomic and cDNA respectively are in lanes 1 and 2 and are identical in size.

Primers Used:

Nucleotide SEQ ID Parent SEQ Position In No. Primer ID No. Parent Sequence 20 2B11-BF: 5′-ttctctactgcgcttctcacatccat-3′ SEQ ID No. 7 2755-2780 21 2B11-BR: 5′-aacatcatagccataagaggtcaaca-3′ SEQ ID No. 7 2963-2988 30 AOKSRT2-F: 5′-ctgacaccatcgaaaacctaaaaa-3′ SEQ ID No. 7 1800-1823 31 AOKSRT2-R: 5′-tctaactcgcccttgacctg-3′ SEQ ID No. 7 2503-2522

To confirm the involvement of the pks gene in OTA biosynthesis, a mutant of A. ochraceus [AO 118] was created which is incapable of producing ochratoxin A, through inactivation of the polyketide synthase (ks) gene. A hygromycin resistance gene cassette was inserted into the (pks) gene on the chromosome of A. ochraceus, so that the pks sequence was interrupted. Disruption of the pks gene was confirmed by the acquisition of hygromycin resistance by A. ochraceus. The hygromycin resistant transformants were then screened by plating on coconut cream agar (CA), OTA is fat soluble and therefore will diffuse into the fat present in the coconut cream. As OTA is fluorescent under UV light, exposure of the plates to UV allows identification of OTA producing strains by the presence of a blue fluorescent ‘halo’ around the culture, identified by the arrow in FIG. 3.

Transformants that appeared to be atoxigenic (OTA negative) after screening on CA were subjected to confirmatory testing by cutting out agar plugs from the medium and extracting the ochratoxin by treatment with acidified chloroform. The extracts [A0118, A0107 and A053] were run on thin layer chromatography (TLC) plates that were visualised under UV light (FIG. 4). From these tests it is clear that the pks gene [7. SEQ ID No. 7 (Polyketide synthase (2179 bp)] encodes a polyketide synthase gene which is essential for OTA production in A. ochraceus.

Isolation of pks Ketosynthase Regions from Penicillium verrucosum and Aspergillus carbonarius.

Genomic DNA was isolated from Aspergillus carbonarius 23804 and Penicillium verrucosum OTA11 and used as a template in PCR reactions with degenerate primers designed from the deduced protein sequence of the ketosynthase region (KS) of the A. ochraceus pks gene. The PCR reactions produced products of the correct size from both fungi. The PCR product was purified by elution from an agarose gel and ligated into the pGEM-T easy vector. The ligation mixture was transformed into Top 10F chemically competent Escherichia coli cells (Invitrogen) and the transformed cells were plated on LB agar containing Ampicillin (100 μg/ml), IPTG and X-GAL. A number of white colonies were selected and screened for the presence of the 2B11 PCR product by emulsifying a portion of the colony in 50 μl of distilled water and using 2 μl of the colony suspension as a template in a PCR reaction with the 2B11-B primer pair (SEQ ID No. 20 and 21). A transformant containing the correct PCR product was selected and grown in LB broth; plasmid DNA was prepared from the LB culture using the Qiagen Spin Mini-Prep plasmid DNA isolation kit. The DNA sequence of the cloned PCR product was determined by Lark Technologies Inc., Saffron Walden, Essex, United Kingdom. The nucleotide sequence was compared to existing nucleotide and protein databases using the BLAST and BLAST-X programs on the NCBI website. Both sequences showed strong homology to the KS region of PKS proteins from a number of fungi, including some known to be implicated in the biosynthesis of polyketide mycotoxins.

Primers Used

KS-DPA: 5′-GCIAAYGGITAYGCIMGIGG-3′ (SEQ ID No. 24) KS-DPB-5′-GTICCIGTICCRTAIGCYTC-3′ (SEQ ID No. 25)

Referring to FIG. 8, no OTA was detected in the culture medium even though we have previously demonstrated OTA production by this strain. Measurement of the pks expression using the GAP primer pair in a RT-PCR assay showed that only a small amount of expression was occurring. This is in contrast to the higher level of pks expression observed in the A. foetidus strain that was producing OTA under these growth conditions (FIGS. 6 and 7).

The PCR primer pairs used for the RT-PCR assay were designed from the A. ochraceus pks DNA sequence. The data in FIGS. 6 and 8 demonstrates their application to the detection of OTA production in other Aspergillus species.

Cloning of a pks Homolog from Aspergillus niger.

Genomic DNA was isolated from Aspergillus niger 9029 and Aspergillus foetius 10254 and was used as a template in PCR reactions with two PCR primer pairs. The primer pair 2B11-B (SEQ ID No. 20 and 21) produced a product from A. niger and the PKS4-GAP primer pair (SEQ ID No. 22 and 23) produced a product from A. foetidus. The products were the same size as those produced by these primers in A. ochraceus. The PCR products was purified by elution from an agarose gel and ligated into the pGEM-T easy vector. The ligation mixtures were transformed into Top 10F chemically competent Escherichia coli cells (Invitrogen) and the transformed cells were plated on LB agar containing Ampicillin (100 μg/ml), IPTG and X-GAL. A number of white colonies were selected and screened for the presence of the 2B11 (SEQ ID No. 20 and 21) PCR product by emulsifying a portion of the colony in 50 μl of distilled water and using 2 μl of the colony suspension as a template in a PCR reaction with the 2B11-B primer pair (SEQ ID No. 20 and 21). A transformant containing the correct PCR product was selected and grown in LB broth, plasmid DNA was prepared from the LB culture using the Qiagen Spin Mini-Prep plasmid DNA isolation kit. The DNA sequence of the cloned PCR product was determined by Lark Technologies Inc., Saffron Walden, Essex, United Kingdom. The nucleotide sequence was compared to existing nucleotide and protein databases using the BLAST and BLAST-X programs on the NCBI website.

Primers Used (all Specific to the pks Gene)

(SEQ ID No. 20) 2B11-BF: 5′-ttctctactgcgcttctcacatccat-3′ (SEQ ID No. 21) 2B11-BR: 5′-aacatcatagccataagaggtcaaca-3′ (SEQ ID No. 22) PKS4-GAPF: 5′-agccgtgttttcattctttc-3′ (SEQ ID No. 23) PKS4-GAPR: 5′-tgcggccatcttcgtgt-3′ Primers Used for Aspergillus

Nucleotide SEQ ID Position In Primer No. Target gene. Target Gene AOB02-F: 5′-tatccgccgcctcgcaaactaat-3′ 10 SEQ ID No. 6 AOB02-R: 5′-cgaccgatcatgcgaccgtaaat-3′ 11 SEQ ID No. 6 AOB03-R: 5′-ctcggtgacatcaggggtatc-3′ 12 SEQ ID No. 5 949-969 AOB03-R: 5′-agcgtattcagtcactcattcaga-3′ 13 SEQ ID No. 5 AOE04-F: 5′-gctatgcgcggagaagtca-3′ 14 SEQ ID No. 2 804-822 AOE04-R: 5′-aaggctggggatcgtggagtg-3′ 15 SEQ ID No. 2 1065-1085 AOD07-F: 5′-agtttaccggccttgttga-3′ 16 SEQ ID No. 4 AOD07-R: 5′-ttattaccgtttgtcgctcttctc-3′ 17 SEQ ID No. 4 AOH11-F: 5′-agaacgggatgccaaaacagtgag-3′ 18 SEQ ID No. 1 AOH11-R: 5′-aagaatgcgagggatgggataacc-3′ 19 SEQ ID No. 1 2B11-BF: 5′-ttctctactgcgcttctcacatccat-3′ 20 SEQ ID No. 7 1610-1629 2B11-BR: 5′-aacatcatagccataagaggtcaaca-3′ 21 SEQ ID No. 7 2963-2988 PKS4-GAPF: 5′-agccgtgttttcattctttc-3′ 22 SEQ ID No. 7 1610-1629 PKS4-GAPR: 5′-tgcggccatcttcgtgt-3′ 23 SEQ ID No. 7 2346-2362 KS-DPA: 5′-GCIAAYGGITAYGCIMGIGG-3′ 24 SEQ ID No. 8 KS-DPB-5′-GTICCIGTICCRTAIGCYTC-3′ 25 SEQ ID No. 8 ACKS-1F: 5′-tgggtatgcgcggggtgagggtat-3′ 26 SEQ ID No. 8 ACKS-1R: 5′-ccgtaggcttcgaaaaactgacac-3′ 27 SEQ ID No. 8 Primers Used for Penicillium

Primer SEQ ID No. Target gene. KS-DPA: 5′-GCIAAYGGITAYGCIMGIGG-3′ 24 SEQ ID No. 9 KS-DPB-5′-GTICCIGTICCRTAIGCYTC-3′ 25 SEQ ID No. 9 PVKS-1F: 5′-tgcacgaccgggacaacatca-3′ 28 SEQ ID No. 9 PVKS-1R: 5′-ccgtaggcctccacaaaatctg-3′ 29 SEQ ID No. 9 Typical PCR Assays Utilising the Invention

The methodology for the PCR assays depends on whether the assay is to detect the presence of fungi that are capable of producing OTA by detecting a biosynthetic gene (PCR) or to detect production of OTA by measuring expression of the biosynthetic gene (RT-PCR).

For the PCR assay a sample such as a grain sample will be ground to break up the grains and DNA extracted from the ground grains using a Qiagen DNeasy plant mini-kit (Qiagen GmBH). The extracted DNA will be used as a template in a PCR reaction with one of the primer pairs listed above. Production of a PCR product of the correct size is taken as a positive result. The PCR assay can be performed on a real-time PCR apparatus such as a Light Cycler™ (Roche Molecular Biochemicals) and this will provide results within 1 hour. The total time from receipt of samples to production of results can be as short as 2-3 hours.

For the RT-PCR assay, a sample such as a grain sample will be ground in liquid nitrogen and the RNA extracted using a commercially available extraction kit such as the RNeasy™ (Qiagen GmBH). cDNA will then be prepared from the RNA by use of the reverse transcriptase enzyme. Once the cDNA has been prepared the procedure is identical to that for the PCR assay.

The invention is not limited to the embodiments hereinbefore described which may be varied in construction and detail.

APPENDIX 1 1. SEQ ID No. 1 (Averantin oxidoreductase (630bp)) GGCCGCGGGAATTCGATTGGCCGCCCGGGCAGGNACTTTTTTTTTTTTTT TTTTTTTCAATTTCAATCTGACCNTNTATTTGNGTGTAGTTGACAGTAAA TCGGAGAATTAATTAAATACAAAATCAGGAGAACGGGATGCCAAAACAGT GAGGCGAAAGAGTGAGGCGACACAACTCAAAACTGCCATCTCCGATTTGA ATCGAACCAAGGAATAAATAATCACCTGCACCCAATTCACAGGAGCAAT GTCACTCTGGGACCTATCCAGGACTGAAACAATAACTCAATAAATCGAT GCTATCTCGTAGCTGCTTGCCCAGTTGGTCCATTCAAACCTGGCGAGTAG ATCATCTAGGAAGCTTTAGGCGTANACTGTGAANAAGAGCGCGCGAGAC AAGTGGTATAACGGGAATAAACACAACATTCCGCACATCCTTACCGATG CACTGGAGTCAACTTCACCTTCAATGGATCCTTTTCCCAGATGTTGTAAG ACTTTTGGTTATCCCATCCCTCGCATTCTTCTTCAAATGCCAGGTCAAAG TTCCACAGTAGCTTCACGGCAATCAGACGCATTTCGGCATAAGCAAGATT CCTTTCCGAGGCAGTTCCCTGGGCCCGTAGGAAA 2. SEQ ID No. 2 (HC Toxin synthase (371 bp)) ACCGCTATGCGCGGAGAAGTCAGCGTGGGCTATTGTCGNTTTTNTTGCGA TTCTGAAAGCTGGAGCTGCCTGTACNCCNCTCGAGCCGTCTCATCCGCGA GATAGATTGAAAAACTTGATTCGAGCCTGTGGTGCAACAGCCGTTGTGGT AACGGCCCCACACGCCTCAATGCTTGAGATGGAGGGAATTGATGTCGTC GTCGTTTCTTCCAAGACTGTATCGTCTCGGGATGAAGGTTTGGCTTGTGT AGCCAATGCCACGGTCACTCCACGATCCCCAGCCTTTCTAATGTGGACAT CTGGAAGCACGGGAAACCCCAAAGGCGTCGTTCTGGAACATGCAGCTCT GTACCTGCCCGGGCGGCCGCTCG 3. SEQ ID No. 3 (Regulatory gene (666bp)) ACCGATGAACATTCCAACGACTCGCCCGAATCGTGGGNTGGCGAGGGCC CCGACAGTTCTCCCGTCGCCGACGTCGAGGTGAAGATCCACGACATTTCC CATCCGCTCGCCATGGCCTCCGCCAAAGCCTTCCACTTCAATATGGTCGC TCCTCACGATGCGGACCTGTCGATGGGAGACGTAACGCCCAAGGTCGAA GAAGTGGACGACGCGGATGATTTACAAAGTATCAAACCTATGGGTGTGG ATCCGCTGGCCAATGCGGACTCTACTCATCTGGATTCCACGGCTCTTCCC GTCAACGTTCCACGGAAACGTGGACGACCACGCAAACATCCTCTTCCAG CTCCCGGAGGTCAAGTGAAAATCACCAAAGGTCGATCCAAGACGGGCTG CATAACGTGTCGACGACGGAAGAAGAAATGCGACGAGACCAAGCCAGC GTGCCTAAATTGTCAGAAAAATGCCGTCGTGTGTGAAGGTTATCCGCCAA AGGAGATCTGGAAGAGTGGGAAACAGAAGCAGGAAGATGCAGCTGCCC GATGCCAGACGATGATCTCTCGTGCCCTCCTTTCTGATCGATGGAATCGA AAGCGATATTGACCGACGCTTTCTGGATCATTTTGGGACCTCGGGCGNGA NCACCTTATCACTAATGAATTTGCG 4. SEQ ID No. 4 (3-oxoacyl synthase (846 bp)) CGAGGAATGCGAACGCTGGGGTTCACTTCGATCAGGNCACCTTTAGCGG TGTGAATGTAGATCTGAAGTTTACCGGCCTTGTTGACGGGCGAATCGAGG AGGGTGAGGAGACACTGATGGGTAATATCCGGTCGAGCCTCGCTGATGT CTCTGTTCATCTTGCGCATCACACCGATATGCTCGTCGCTGTTGAGCAAG GAGTATTTCTCATCACGGTTGGTGCCGCTGCGTCCGCCATGAGAGGCGCG GAAAGTTTCCAAGCTTGCATGAGACAGGACGACGATCAAGCGCTGGGTC TCTTTATCGTGGGCAGAGATGGGGACATGTTGCTCGGCGACCAACTGAG GCAGTTCGGGAGGAGGGCAAGACTGGGTCCTGGCACGCTTTCTCCTTCCT GCTACTTGAACAGAATCCGACATATTGAACCGATGAACCAATTGAAGAC TTTTTTTATATAAAATAAAAAAGGAGAGTATTATATGTAATTCGAGACTC GGGACCGATAAGAAGAATCGAGGGAGGAGGAGAAAAGACNGACGACAG ACGATTGAGAAGAGCGACAAACGGTAATAAAGGAAGATCCCGAGAGGC GCANAGAGAANGAAGCGGTTGCAGGATAACTGCAGTCCANGATGANGG GATTCAAGGGGGATGTTTGTCGCTTGCTTAACCCCGGACTGCCGCGCGTC AATATATGATCCGCGCTGAGCACTTGAANTCACCGTGANTANTGGTTTGG ACTAANTGNGAATGNANTGTCTGGAATGTCTCATCCCAANACGACTAAG AGCGGNATNGCATCATATGTGNNTCNTCACGACGGCATTATACCACCGG AGGTGTG 5. SEQ ID No. 5 (Trichodiene oxygenase (572 bp)) CAGACTGTCATTGTATGGGCTGTCGTTTCAGTTCCCNTTNNCACAATGGT AATGTATTCATGAAGAACCCTANCCTCGGTGACATCAGGGGTATCGGGG GCTTTGGCGACCATTTCCTGGAAAAGCGCCGGTTCCTGGGAGAGTCTGTT TTTCTCAAAGTCTAGCTTCGCCTCTTCTTCGTTCTCCCACATGAAGTTCA CCTCCTGGGCACATTTCATTCGAAATGCAATCACAGGGAGCAACTTGGGA TAAACCCGCTTGACCCACTTCATTCCCATGCTCGCCAGAAGAGGGAAAAA TCCCGGAAGATGCCGGGCGATCATTCCGATCTCGACGAGGTTCTTAATTG TTTCACTCCATATAGGGACCATCTCGGGGTCGTCTAGGTAGTTGAACGCA CGGAAGCTCGTATAAGAGGTGATAATGTCGGCGGTGAAACAGTTGAATG CGACGTGTACTTTTAGGGGCTCCCCCGAGTGAGCATACTCGCTCAACCGG GTATTCAGTTTTGAAGTTTACTCTGAATGAGTGACTGAATACGCTGTAC CTCGGCCGCGACCACGCTAATCACTAGTGAATTC 6. SEQ ID No. 6 (Acyl CoA dehydrogenase (670 bp)) ACTATCCACTTGATGGAATTCACAACTTCTTCTCCCTGTTTCCCTCTCTA AAGCCAAAAACAGACCCCGCAATCTTTCTCATCTGGATTTCTTCCGACCC CTCCGTAATTCGATACCGTCTAAAATGACGATAAATGTGCTCGAATAGAT AATGGCGCGAGTATCCATCTCCCCCGTGGATCTGAATCGCCCTATCCGCC GCCTCGCAAACTAATCTATTAGCCCAAAAATTACACATTGCGACCTCATC GCCCAGTTTCTTCTCGATCTCCACCCAAGGCTTTCTCCCCTCGCTCTCCG CCTCCCCAGCTACCTGATCCATGTCCACACTAGTCTTCAGAATCAGCAGT CGGAGCATCTCGACCTGCGTCACCAACTCAACGACTGGGAACTGAATGCC CTGGTTCTCGGACAGACCCTTTCCGCCCCAAATTTTCCTGTTCTTCGCGC GCTCAATGCTCCGATCCAGGCAGAACTTTGCCGCACCGCATGAATTGGCC GCCTGTCGCAGTCGATTTTCATACATGAATGTCTGCGCGATCGCTAGTCC CTCGCCAACAGTTCCTAGCACAGCCTCAATTGACACAAAAACCCGATCTA AATTTACGGTCGCATGATCGGTCGGCATATTCATCGGCCATTCGTAACTA CGATTTCACCCCCTTCGTCT SEQ ID No. 7 (Polyketide synthase 2179 nucleotides): TCGAAGGCAATTGCCAATGGAGAATCAAATGGCACAAGTCGCAATTCCAC TTCGCAGTTGGATCCCGGAAAGCCGTGTTTTCATTCTTTCGGCATTTGAT GAGGCTGGTTTGGACCGAAACGCGATGTCCATGATATCTTACCTCGAATC ATTGAAACTCTCCGGAGATCCTGATCTGGAAGAAGCGTTCATGAGCGACC TATGTCATACACTAAACGCAAAACGCACAATGTTTGATTGGCGCAGCTAC CACGTCGCTGACACCATCGAAAACCTAAAAAAGTCGCTTCGGAATATCCG CCCATATCGTCGATCGACAAGCTCGAAAGCCGTCCGTTTTATATTCACGG GCCAAGGAGCGAATTGGGCTGGCATGGCCCAAGATCTCTTACTTTACCCA TTGTTTCGACAACGGATCCAAGAAGCTGCAATGTTCTTGGGAGAAATTGG ATGCGAGTGGGATCTTTATGGTATAGTCTTGTCCATTCCTTTTCTTAATC CTTTTTTTTGTCTTTATTTCCTCCTCTCTCCCCTCAAATTATTGGGTGAC GCCGAATAACCAAAAATTTAGACCGAATCAGCTCACAGCACGGTGACCTA AACGAGCCCACTTTTGCGCAATCCTCCTGTGTAGCCGTGCAGATTGCGTT GGTAGACTTGCTTCATAGCTGGAAGGTGACGCCAACAACGGTAGTAGGAC ATTCGTCCGGCGAAATTGCAGCAGCGTATTGTGCGGGAAAAATCTCACGT CAAGCAGCATGGAAAATCGCCTACTGTCGAGGACAGGTTTGTGCAAAACA GACACACGAAGATGGCCGCATGCTCGCAGCGGCTATGCCTGCACAGGAA TTGGAACGACTGTTAGCTCGTTTGAACAAAGGTCTATGCTCTGCGGTTCA GGTCGGATGTTATAACAGCCCCAAGAACTTGACCCTGACAGGCCAACAC GAGAGCATTCTTCAGGTCAAGGGCGAGTTAGACGAAGCGGGTGTGCTAA ACCGTTTACTTCCAGTTAAGGTTGCTTATCACTCGAAATTCATGCGAGAA GTCGCTCCAGAGTACTTAGAGCTCCTCGGGGACCTGGATTTCGGTGACAA GATGACCGACCATGCCAAAGTTACTATGATATCCTCGGTTACGGGACGAC ATGCACTCGCAGGGGAGGTTGAGAGTCCTTCGTACTGGGTTGATAATCTG ATCTCACCTGTCCGTTTCTCTACTGCGCTTCTCACATCCATGCAAACACA AAGTCAAAAGTCACCCAGCGATAACGCACTGATTGAAATCGGACCTCATT CTACTCTCCGCACCGCCATCAATGAGACCCTTGCGGATCAACCTACACTG CAGCCGTTTCAATACGGTAGTCTGCTCAAGCGATATGAGACTGACGGAA CGACAAGCCTGCGCACATTCGATTTGTTGACCTCTTATGGCTATGATGTT AGCCTGGCTTCTGTTAATGATCCTCGATCAAAAATTAAGAAAGCTCCTCA TATGATAACGGATCTTCCGCCTTACTCCTTTGACCACTCACGGTCGGTTC GCGGCCAGTCTCGAAGAATCAAAAACATCAAGTTTCCAGCGTACGAACGC CATGAGCTCCTTGGTGCGCCAGTTGAAGATACAAACAAATTTGAGCAAC GATGGAGGAACATCATTAGACCGGACGATATTACATGGTTGCGCATGAA CAGAGTGAGTACTTCCTATAAAATTATGAGCCCATCTAACGTGAGCCAGA TGGATGGAAGTATCCATTTCCCCGGAGTCGCCTACCTTTTGATGGCCATG GAGGCCATAATGCAGCGAACTGGGATGACAGAATGTGTCACCGGCATTA GGATTGGCAATGTGGCTATGCTGGCCCCTTTGCCTGTTCCCGACACCCCA GAAGGTGTCGAGATCATCTTTTCGATCTACCCGATGAACGAGTCAGCCCG GGCCACAGATGACTGGTGCACCTTCAGAGTTATCTCCCATGAAGGGGTTG AAAATTCTTGGATTGAGCACTGCGTTGGTTCAGTTCGTATAGAAACGGGA GAGCAGAGGATATCCGCCCCTCCTGTTGACAGCCAGTTGTCAATATGTTC TGAAGCCGTCGATATAAACCAAATGTATCGAGACTTCGCCTCTGCGGGA ATGGAATTTGGCGACTTCCTGAAAAACATTCGAAGCAT SEQ ID No. 8 Aspergillus carbonarius ketosynthase (KS) region of the pks gene sequence (314 nucleotides): GAATTCACTAGTGATTGCGAATGGGTATGCGCGGGGCGAAGCCATTGGC TGTCTAATACTCAAGCCATTGAAGAATGCTGTTCGAGATGGGGATCATAT TTACGCTATCATCCGAGGATCAGGGTCTAACCAGGACGGGAGAACCCCC GGAATCACGCTCCCTAGTGAGGTGGCGCAAGAGGCTTTGATACGACGCG TATATCAAATGGCATGTCTTAATCCAGCAGATACCGACTTCGTTGAGGCC TACGGCACCGGCACAATNGAATTCCCGCGAGCCGCCAGGCGGCCGGGAG CTGGAACTCGGGCCCATG SEQ ID No. 9 P. verrucosum ketosynthase (KS) region of the pks gene sequence (311 nucleotides): CATGGGCCCGAGTTCCTGCTCCCGGCCGCCTGGCGGCCGCGGGAATTCCA TTGAATGGGTATGCGCGGGGCGAGGCAGCTGGCTGTCTTATCCTTAAACC CCTAGCCAAGGCGTTGCACGACCGGGACAACATCAGGGCCGTAATACGA GGAACCGGTTCCAATCAAGACGGGCGCACCGCAGGGATAACACTACCAA ATGGGGCAGCCCAAGAAACCTTGATTCGGAGCGTCTATACACGGGCTGG TCTGGATCCCTCCGAAACAGATTTTGTGGAGGCCTACGGCACCGGCACAA TCACTAGTGAATTC

REFERENCES

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1. An assay for the detection of an Aspergillus ochraceus strain producing ochratoxin A comprising the steps of: obtaining a sample; extracting DNA from said sample; using said extracted DNA as a template to amplify DNA sequences in the presence of an oligonucleotide primer derived from the PKS nucleic acid sequence of SEQ ID No. 7; and analysing the amplified sequences for the presence of the PKS nucleic acid sequence of SEQ ID No.7 wherein an Aspergillus ochraceus strain producing ochratoxin A is identified by the presence of a PKS nucleic acid sequence comprising SEQ ID No.
 7. 2. The assay of claim 1 wherein the oligonucleotide primer is selected from any one or more of SEQ ID No. 20, SEQ ID No. 21, SEQ ID No. 22 and SEQ ID No.
 23. 3. An assay for identifying an Aspergillus ochraceus strain expressing a PKS gene comprising the steps of: obtaining a sample; extracting RNA from said sample; synthesising complementary DNA (cDNA) from said extracted RNA; using said extracted cDNA as a template to amplify cDNA sequences in the presence of an oligonucleotide primer derived from the PKS nucleic acid sequence of SEQ ID No. 7; and analysing the amplified sequences for the presence or absence of the PKS nucleic acid sequence of SEQ ID No. 7, wherein an Aspergillus ochraceus strain expressing a PKS gene is identified by the presence of a PKS nucleic acid sequence comprising SEQ ID No.
 7. 4. The assay of claim 3 wherein the oligonucleotide primer is selected from any one or more of SEQ ID No. 20, SEQ ID No. 21, SEQ ID No. 22 and SEQ ID No.
 23. 5. An assay for the detection of an Aspergillus ochraceus strain producing ochratoxin A comprising the steps of: obtaining a sample; extracting DNA from said sample; using said extracted DNA as a template to amplify DNA sequences in the presence of a pair of oligonucleotide primers wherein said pair of oligonucleotide primers are selected from the group consisting of (i) the pair of oligonucleotide primers of SEQ ID No. 20 and SEQ ID No. 21, and (ii) the pair of oligonucleotide primers of SEQ ID No. 22 and SEQ ID No. 23; and analysing the amplified sequences for the presence or absence of the PKS nucleic acid sequence of SEQ ID No.7 wherein an Aspergillus ochraceus strain producing ochratoxin A is identified by the presence of a PKS nucleic acid sequence comprising SEQ ID No.
 7. 6. An assay for identifying an Aspergillus ochraceus strain expressing a PKS gene comprising the steps of: obtaining a sample; extracting RNA from said sample; synthesising complementary DNA (cDNA) from said extracted RNA; using said extracted cDNA as a template to amplify cDNA sequences in the presence of a pair of oligonucleotide primers wherein said pair of oligonucleotide primers are selected from the group consisting of (i) the pair of oligonucleotide primers of SEQ ID No. 20 and SEQ ID No. 21, and (ii) the pair of oligonucleotide primers of SEQ ID No. 22 and SEQ ID No. 23; and analysing the amplified sequences for the presence or absence of the PKS nucleic acid sequence of SEQ ID No.7 wherein an Aspergillus ochraceus strain expressing a PKS gene is identified by the presence of a PKS nucleic acid sequence comprising SEQ ID No.
 7. 